Acrylic acid is an important industrial chemical that is used in multiple applications in the polymers industry as well as in other contexts, and acrylic acid and its esters (acrylates) are produced globally in the hundreds of thousands of tons annually. Acrylic acid is conventionally produced by oxidative processes and a variety of feedstocks are known from which acrylic acid may be produced by single or multistage oxidations. Most acrylic acid is produced commercially from propylene, but with increased supplies of inexpensive and abundant natural gas in recent years, propane has attracted increased interest as a feedstock for direct conversion to acrylic acid as have biobased or renewably sourced materials.
With respect first to the biobased or renewably sourced feed-based routes, a number of references describe methods for converting glycerol to acrylic acid and/or acrylates, commonly using glycerol such as that produced in the making of biodiesel (fatty acid methyl esters) from plant oils, see, e.g., U.S. Pat. No. 7,396,962 to DuBois et al. and references cited therein.
A number of efforts have likewise been made to develop processes for making acrylic acid from carbohydrate and/or carbohydrate-derived feedstocks. One feedstock that can be derived from carbohydrates and that has been closely evaluated is 3-hydroxypropionic acid, or 3-HPA. U.S. Pat. No. 2,859,240 to Holmen (1958) indicates that the dehydration of 3-HPA is a “comparatively simple and economical process”, but concludes that “the starting material is neither low in cost or readily available in quantity” (col. 1, lines 55-58) Essentially the same assessment is offered 45 years later, wherein in Kumar et al., “Recent advances in biological production of 3-hydroxypropionic acid”, Biotechnology Advances, vol. 31, pp. 945-961 (2013), the authors conclude despite “significant progress” in the preceding decade toward “commercial production . . . in the near future” that “many important issues still remain and require more extensive investigations.”
Another feedstock that can be derived from carbohydrates and that has been the subject of considerable research as well is lactic acid. In the same 1958 Holmen patent, for example, lactic acid is indicated as having been recognized for some time as preferable to 3-HPA as a prospective feedstock due to its ready availability (referencing a 1950 review of efforts to that time to develop processes for converting lactic acid and the lower alkyl esters of lactic acid to acrylic acid and the corresponding lower alkyl esters of acrylic acid). A commercially viable process yet remains elusive as well for the conversion of lactic acid to acrylic acid, as evidenced by a number of ongoing applications for patent that have recently been filed.
WO 2012/033845 to Ozmeral et al, WO 2012/156921 to Dongare et al. and WO 2013/155245 to Lingoes et al. are representative of these ongoing efforts to develop a commercially viable process for converting lactic acid (and/or lactate esters) to acrylic acid (and/or the corresponding acrylate esters), and each in turn reviews a fairly substantial body of additional published art detailing prior work toward the same objective.
In WO 2012/033845, a fermentation broth containing ammonium lactate is described as processed according to one of four pathways to produce acrylic acid esters. In a first pathway, lactic acid is first purified from the fermentation broth. The highly purified lactic acid is then subjected to a vapor phase dehydration reaction at elevated temperatures and in the presence of an appropriate catalyst to produce acrylic acid, which in turn is esterified in the presence of an esterification catalyst to provide the acrylate esters. In a second pathway, lactic acid in the fermentation broth is dehydrated “without much purification”, followed by an esterification to produce acrylic acid esters. In the third pathway, ammonium lactate in the fermentation broth is subjected to simultaneous dehydration and esterification reactions to produce an acrylic acid ester product, while in the fourth pathway, ammonium lactate in the fermentation broth without much purification is subjected first to an esterification reaction to produce a lactic acid ester, and then this lactic acid ester is dehydrated to provide an acrylic acid ester product. In a “most preferred” embodiment according to this fourth pathway, a fermentation broth containing ammonium lactate is concentrated by evaporation of water and subjected to esterification with a C1-C10 alkyl alcohol, preferably in the absence of any exogenous esterification catalyst. Ammonia released during the concentration process is captured for recycling to the lactic acid fermentation, along with further ammonia released during the esterification reaction. The lactic acid ester obtained in the first stage is then dehydrated to produce a corresponding acrylic acid ester.
In WO 2012/156921 to Dongare et al., a catalyst with improved selectivity to acrylic acid from lactic acid and reduced production of acetaldehyde and other products is offered for use in the dehydration of lactic acid to acrylic acid, comprising a calcium phosphate in a calcium to phosphate ratio of from 1.5 to 1.9 as optionally modified with 5 weight percent of sodium. The process is described as involving preheating the catalyst in a fixed-bed reactor at a temperature of 370 to 380 degrees Celsius for from 20 to 40 minutes under highly pure nitrogen, then passing 50-80 wt. pct preheated vapors of a lactic acid solution through a quartz fixed catalyst bed reactor by means of a nitrogen carrier gas. Reported lactic acid conversion under these conditions was 100 percent, with 60 to 80 percent selectivity for acrylic acid and about 15-35 percent selectivity for acetaldehyde.
In WO 2013/155245 to Lingoes et al., reference is made initially to research by a number of parties of a similar character to that reported in Dongare et al., which research confirmed that phosphate and nitrate salts may desirably change the surface acidity of acidic catalysts to inhibit the decarbonylation/decarboxylation of lactic acid to acetaldehyde in particular.
Lingoes et al. contend that even with a reduced selectivity to acetaldehyde, nevertheless even the reduced amounts are problematic, as byproducts can be deposited on the catalyst and result in fouling and in premature and rapid deactivation of the catalyst. Further, once deposited, the byproducts can catalyze other undesired reactions, for example, polymerization reactions (para.0005).
As well, apart from the difficulties caused by being deposited on the catalyst in question, Lingoes et al. point out the difficulties even very small amounts of byproducts such as acetaldehyde, propanoic (or propionic) acid, carbon monoxide, carbon dioxide, 2-3-pentanedione and lactic acid oligomers can cause in processing acrylic acid from the then-known lactic to acrylic processes to make superabsorbent polymers, such that a significant body of literature existed around removal of these impurities from the acrylic acid.
Lingoes et al. reference U.S. Pat. No. 6,541,665 and US Published Pat. Appl. 2011/0257355 as exemplars of this body of literature. In U.S. Pat. No. 6,541,665, a 5-stage crystallization (containing two purification stages and three stripping stages) was effective to obtain 99.94% acrylic acid containing 2600 parts per million by weight of acetic acid and 358 ppm of propionic acid, among other species. In US 2011/0257355, a method is described of removing propionic acid in a single pass crystallization from a crude reaction mixture derived from glycerol dehydration/oxidation to obtain 99% acrylic acid. According to Lingoes et al, prior to their improved catalyst and process, the prior art methods for converting lactic acid to acrylic acid produced amounts of byproducts that were too high (“far too high”) to even utilize such purification methods.
With respect to the use of propane rather than propylene as a feed for the production of acrylic acid, U.S. Pat. No. 7,795,470 to Dieterle et al. provides an example of the further work that has been undertaken on this particular alternative approach, and contains a lengthy review of still earlier efforts.
In Dieterle et al, a method is described for the heterogeneously catalyzed partial direct oxidation of propane to acrylic acid in the gas phase, in which n-propane, molecular oxygen and at least one inert diluent gas but including not more than 1 mol percent of cyclopropane are fed to a reactor and passed through a catalyst bed comprising a catalyst in the solid state of aggregation to convert n-propane to an acrylic acid product, and the acrylic acid product is purified by at least two separation zones. In a first separation zone, acrylic acid present in the gaseous product from the reactor is converted to the liquid phase, and the remaining gaseous residual product gas mixture comprising n-propane and depleted in acrylic acid is removed from the first separation zone for recycling at least a part of the n-propane to the reactor, while acrylic acid is separated from the remainder of the liquid phase from the first separation zone in a second separation zone using at least one thermal separation method which comprises at least one isolation of acrylic acid by crystallization. Dieterle et al. explain that propane as produced from natural gas processing includes other constituents that can account for up to 10% by volume and more of the total volume of the crude propane, referencing WO 2006/120233 to Diefenbacher et al., which itself describes certain methods for removing constituents that cause difficulties in the conventional propylene oxidation processes. Dieterle et al. purportedly found that cyclopropane, though an isomer of propylene and behaving like propylene chemically in certain contexts, nevertheless formed propionic acid in their propane to acrylic acid method and could be accommodated only to a limited extent.
As is immediately evident from the foregoing review of the prior art, one challenge common to each of these propylene-alternative methods—whether employing propane or one of the biobased feed-based alternatives—is the presence in the crude acrylic acid products provided by these various alternative methods of assorted impurities.
Acetic acid and propionic acid are recognized as particularly problematic in that both are saturated and cannot be polymerized, so that depending on the polymerization process involved and the applications targeted for the polymer, these impurities may remain in the finished product and risk conferring undesirable corrosive properties on the finished product or being reencountered as waste in the liquid or gaseous discharges from the polymerization process. Commercial acrylic acid processes proceeding from propylene via a two stage oxidation typically yield propionic acid concentrations of not more than approximately 1,000 parts per million by weight. Unfortunately, however, acrylic acid produced from glycerol typically contains at least about five times this much propionic acid (according to U.S. Pat. No. 8,440,859 to Dubois, assigned to Arkema France) and acrylic acid from propane may contain from three to thirty times the amount produced in a typical propylene-based process (according to EP 2039674 B1 to Han et al., assigned to Rohm and Haas Company). For acrylic acid from lactic acid, Lingoes et al. claim to have been the first to produce acrylic acid from lactic acid with low enough byproducts for further purification methods—costly as they might be, for example, multistage crystallization—to be sufficient to achieve a commercially acceptable glacial acrylic acid purity, yet Lingoes et al. describe their process as being preferably “sufficient” “to produce propanoic acid in a molar yield of less than about 6%, more preferably less than about 1%” and in fact show a substantial deterioration in performance in a number of respects, including byproduct make, unless quartz reactors were used.
Further, while additional acetic acid from glycerol- , lactic- or propane-based processes can be removed to an extent in a light fraction by conventional distillation methods, the boiling points of propionic and acetic acids are virtually identical so that separation by distillation of the excess propionic acid is not really possible. As well, propionic and acrylic acids have similarly solubilities in the solvents that have commonly been considered for solvent extraction, so this method has heretofore not been successfully used as a means to separate the propionic acid byproduct from the crude acrylic acid product to make a glacial acrylic acid product of acceptable purity.
According to the above-mentioned EP 2039674 B1 to Han et al, claiming priority from September 2007 and published as of Mar. 25, 2009, the only commercial technique available for effectively separating propionic acid from acrylic acid is melt crystallization as described in U.S. Pat. No. 5,504,247 to Saxer et al. (Sulzer Chemtech AG). This approach is however very capital- and energy-intensive. An alternative approach acknowledged by Han et al. would involve a selective reduction of propionic acid in the presence of a catalyst, but the one example acknowledged by Han et al. from the prior art (from JP 2000053611) concurrently showed an undesirable yield loss of up to 8.6 percent of acrylic acid that was oxidized in reducing the propionic acid content from 337 ppm to 115 ppm, using a MoFeCoO catalyst.
Han et al. propose an improved catalyst and method for the selective reduction of propionic acid, wherein a mixed metal oxide catalyst of the formula AaMbNcXdZeOf is used, where A is “at least one element selected from the group consisting of Mo and W; M is at least one element selected from the group consisting of V and Ce; N is at least one element selected from the group consisting of Te, Sb and Se; X is at least one element consisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Sb, Bi, B, In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu; and Z is at least one element selected from the group consisting of Zn, Ga, Ir, Sm, Pd, Au, Ag, Cu, Sc, Y, Pr, Nd and Tb; and O is oxygen in oxide form and wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0, e=0 to 0.1, and f is dependent on the oxidation state of the other elements”. Preferred catalysts were “MoaVmTenNbxOo and WaVmTenNbxOo wherein a, m, n, x and o [sic—f?] are as previously defined”. Han et al.'s catalyst exhibited reduced acrylic acid yield losses, but the losses were still substantial, being “only” about 6 percent.
US Published Patent Applications 2013/0267735 and 2013/0274509, both to Han et al. and both claiming the same priority and PCT filing dates, briefly describe additional methods for reducing the amount of propionic acid produced in the process of oxidizing propane to acrylic acid. In the former, a reduced amount of steam is employed to reduce the amount of propionic acid produced in the oxidation. While steam is acknowledged as necessary for the economical conversion of propane to acrylic acid, and while “some” acrylic acid yield is lost with reduced steam levels, yet Han et al observe that “the economic benefit derived from reducing the capital and operating costs of the process required to separate PA [propionic acid] from the final product can offset the yield loss.” The latter filing indicates that the amount of propionic acid produced in the oxidation can also be reduced by using a reactor with an L/D ratio greater than 10, and preferably in the range of from 20 to 40, and by carrying out the process virtually isothermally. The example of the '509 application more particularly shows a reduction in propionic acid make from 1500 ppm to 950 ppm by employing a reactor (0.25 inch OD) that was 4 times as long as the reactor for a comparative example, and operated within a two degree temperature range throughout its length as compared to a 42 degree range for the comparative example. Interestingly, the latter filing contains no similar concession of acrylic acid yield loss, though the sole example provided in the '509 application uses a reduced amount of steam in line with the '735 application and in fact appears to directly correspond with Example 2 of the '735 application, wherein reduced propionic acid make is said to be achieved with some sacrifice of acrylic acid make.
The above-referenced U.S. Pat. No. 8,440,859 to Dubois adopts an entirely different approach in view of the significant additional production of propionic acid in Arkema's process for making biobased acrylic acid from glycerol, describing their solution as a method for producing a bioresourced propionic acid product from glycerol that is sufficiently concentrated to be used in the same applications as propionic acid from a propylene-based process, see col. 3, lines 50-54. Cited applications include use as a solvent, as a food preservative, in herbicide manufacture and in the preparation of vinyl propionate monomer for certain polymer applications, col. 1, lines 15-19. In Dubois's method, the mother liquor from a melt crystallization purification method of the type described in U.S. Pat. No. 5,504,247 is hydrogenated to convert acrylic acid retained in the mother liquor to propionic acid.
A better solution is clearly needed, particularly in the context of processes for making biobased acrylic acid (whether from glycerol, lactic acid or some other material or combination of materials) or for making acrylic acid from propane, wherein excessive propionic acid is generated or wherein acceptable propionic acid generation requires concessions in the amount of acrylic acid that can be made. “Biobased” as used herein, it should be noted, means and refers to those materials whose carbon content is shown by ASTM D6866 to be derived from or based in significant part (at least 20 percent or more) upon biological products or renewable agricultural materials (including but not being limited to plant, animal and marine materials) or forestry materials. “Wholly biobased” thus will be understood as referring to materials whose carbon content by ASTM D6866 is entirely or substantially entirely (for example, 95 percent or more) indicated as of biological origin.
In this respect ASTM Method D6866, similar to radiocarbon dating, compares how much of a decaying carbon isotope remains in a sample to how much would be in the same sample if it were made of entirely recently grown materials. The percentage is called the biobased content of the product. Samples are combusted in a quartz sample tube and the gaseous combustion products are transferred to a borosilicate break seal tube. In one method, liquid scintillation is used to count the relative amounts of carbon isotopes in the carbon dioxide in the gaseous combustion products. In a second method, 13C/12C and 14C/12C isotope ratios are counted (14C) and measured (13C/12C) using accelerator mass spectrometry. Zero percent 14C indicates the entire lack of 14C atoms in a material, thus indicating a fossil (for example, petroleum based) carbon source. One hundred percent 14C, after correction for the post-1950 bomb injection of 14C into the atmosphere, indicates a modern carbon source. ASTM D6866 effectively distinguishes between biobased materials and petroleum derived materials in part because isotopic fractionation due to physiological processes, such as, for example, carbon dioxide transport within plants during photosynthesis, leads to specific isotopic ratios in natural or biobased compounds. By contrast, the 13C/12C carbon isotopic ratio of petroleum and petroleum derived products is different from the isotopic ratios in natural or bioderived compounds due to different chemical processes and isotopic fractionation during the generation of petroleum. In addition, radioactive decay of the unstable 14C carbon radioisotope leads to different isotope ratios in biobased products compared to petroleum products.